Optochemical detection strategies for heavy metals in water

Abstract

Groundwater contamination by toxic heavy metals is a serious global issue, therefore, there is an increasing demand for fast, portable and reliable on site monitoring methods for heavy metals in water. Conventional laboratory based methods are not capable of meeting this demand as they require expensive instrumentation and highly trained technical staff. Consequently, cost effective and user friendly alternative methods are needed. Microfluidic detection devices have been employed for routine monitoring of water quality parameters such as nutrients, however, a limited number of commercially available techniques are available for heavy metal monitoring in water. Although numerous examples of optical methods for heavy metals have been described in the literature, only a small number of these methods have been successful in real applications. The aim of this research was to develop an optical method for heavy metal monitoring using microfluidic detection systems, and thereby enhance the range of available techniques for water quality analysis. An extensive literature review was carried out to identify candidate optochemical based heavy metal detection methods which could be further optimised and integrated into microfluidic detection systems. Preliminary screening was carried out in the laboratory using UV-vis spectroscopy to assess different optochemical method suitability for application in microfluidic detection systems. Micro scale quartz cuvettes were used to replicate the restricted path length in microfluidic detection chip. For chromium detection in water, a 1,5-diphenylcarbazide method was assessed. Parameters such as colour stability, reaction time, reagent stability, the effect of interfering ions, linear range, and limit of detection were investigated. Additionally, the method’s effectiveness to monitor the target analyte in environmental water samples with various matrices was evaluated. A strong analytical signal was obtained from experiments carried out in micro scale quartz cuvettes. In addition, simple reagent to sample ratio was obtained by combining the reagents, which in turn enables cost effective microfluidic detection system design. The method showed great potential for use in microfluidic detection system. For arsenic monitoring in water, methods based on leucomalachite green, variamine blue, and molybdenum blue were assessed. Similarly to the chromium method’s assessment optimum reaction conditions, reproducibility, colour stability, linear range, and limit of detection were determined for the different arsenic detection methods. The leucomalachite green method was chosen for integration into microfluidic detection systems due to its fast reaction time, strong colour development, and ability to detect arsenic in various environmental water samples. The analytical system used was based on an existing microfluidic platform developed by project partners TE Laboratories, with appropriate revisions as required to incorporate the optimised optical method. Polymethyl methacrylate microfluidic detection and mixing chip was designed for arsenic detection using leucomalachite green method. The microfluidic detection system’s design was optimised in order to enhance the reagent and sample mixing efficiency. LED and photodiode were coupled to the detection channel and served as miniaturised UV-vis photometer. Syringe pumps were used for sample and reagent introduction. A range of spiked arsenic samples were analysed using the microfluidic detection system. In addition, the effect of iron interference on arsenic monitoring was investigated. Linear range and limit of detection for the microfluidic detection method were determined. As a result, a novel arsenic determination method based on microfluidic detection was developed. Although the method’s linear range was too high to be used for arsenic determination in most environmental waters, it showed a great potential for arsenic monitoring in ground or surface waters with known high arsenic concentrations as well as in waste waters.